An apparatus of this type is known from DE 22 31 598/A1, in which an apparatus and process are disclosed for determining the surface tension at the interface between liquids and gases. The apparatus uses a capillary tube for supplying gas with a connector and a nozzle. The capillary tube extends from above down into the container which receives the liquid. For measurement in molten metals the capillary tube is exposed to the heat which rises from the liquid to be measured. The capillary tube is mounted outside of the crucible. Such an arrangement is rather expensive.
A further apparatus of this type is known from DE 29 15 956/A1, in which is described an apparatus for measuring the surface tension of electrically conductive liquids. This apparatus has a capillary tube with a connecting sleeve and a nozzle. The end of the capillary tube that carries the nozzle is bent in a U-shape. The capillary tube is immersed from above in a liquid so that the nozzle is pointing in an upward direction. During operation of the apparatus, gas bubbles exit the nozzle and rise vertically inside a measuring tube. Two electrodes supplied with a voltage and connected to a time-keeping device are arranged on the measuring tube. When the gas bubbles pass between the electrodes, an interruption in the current flowing between the electrodes is brought about; the frequency of the interruptions that the gas bubbles cause in the electrical circuit is measured. The capillary tube is supplied with a flow of gas at a constant pressure, so that by making use of the frequency of the gas bubbles, the surface tension of the liquid can be determined.
Apparatus of this type are relatively complicated, since the capillary tube immersed in the liquid from above must be additionally mounted, like the measuring tube that has the electrodes. In conjunction with that, these elements must at the same time be protected from the increasing heat from molten metals, for example. Even the necessity for generating a flow of current within the liquid requires a relatively high expenditure for operation and safety. The danger of possible leaking currents also has to be viewed as a problem, if the gas bubbles do not perfectly insulate the electrodes from one another, since the results of the measurement can be distorted as a result of such current leaks.
Further, an apparatus is known from DE 42 28 942/C1 for measurement of the surface tension in liquids, wherein a capillary tube is partially arranged in a crucible for receiving the liquid, and wherein the capillary tube extends through the wall of the crucible. The gas to be conducted into the liquid flows through the capillary tube by way of a gas distribution unit. The gas flows out over the entire surface of the gas distribution unit, more or less irregularly, and thereby reaches the surface of the liquid under constantly changing conditions, where a sampling device catches a portion of the gas bubbles (as a rule the largest) and leads these as a measuring impulse to an analysis. Due to the different outlet openings, the different paths of the gas bubbles to the liquid, and due to the different size gas bubbles emitted by the distributing unit, as well as due to the inexactness of the receiving of the sampling device arranged over the liquid, an exact measurement of the surface tension with the described apparatus is not possible, since such an apparatus as a rule will not completely and correctly catch the gas bubbles which escape from the liquid at different places and in different sizes.
An additional apparatus for measurement of surface tensions is described in EP 0 149 500. Also, the determination of the frequency of gas bubbles, here in liquid pig iron, is described in G. A. Irons and R. I. C. Guthrie "Bubble Formation at Nozzles in Pig Iron," Metallurgical Transactions B, Volume 9B, pages 101-110, March 1978. Shown here is an apparatus in which the gas bubbles are detected by means of a microphone.
By making use of the surface tension, apparatus of this type are used, by way of example, to determine the properties of molten metals. Knowledge of the surface tension of molten cast iron makes it possible, among other things, to draw conclusions concerning the graphite morphology of the carbon contained in the cast iron, since the surface tension and the interfacial energy between various phases influence the microstructure of an alloy. This effect is described in the article by E. Selcuk and D. H. Kirkwood, "Surface Energies of Liquid Cast Irons Containing Magnesium and Cerium", Journal of the Iron and Steel Institute, pages 134-140, February 1973. Admixtures of cerium and magnesium with cast iron accelerate the formation of spheroidal graphite, that is, with increasing content of cerium or magnesium, the form of the graphite crystals changes from the lamellar type of graphite at the beginning to the spheroidal type (spheroidal graphite), which is sought in the practice of casting, because a graphite morphology of this type generates optimal strength properties in the cast iron.